New mechanisms for UE specific assignments of uplink demodulation reference signal, DMRS have been standardised in the 3rd Generation Partnership Project, 3GPP, Long Term Evolution, LTE Release-11. This enables DMRS coordination to achieve UpLink, UL, DMRS orthogonality between UEs belonging to different cells or scheduling nodes. By coordinating the DMRS parameters, inter-cell DMRS interference may be reduced.
However, inter-cell coordination requires fast connection, corresponding to low backhaul latency, between scheduling nodes implementing coordination of DMRS parameters. Such requirement may be infeasible for many deployments characterized by too high backhaul latency and/or jitter.
Another hinder to DMRS coordination is scheduler complexity, where scheduling decisions need to be jointly determined among the coordinated scheduling nodes. Scheduling is a complex task even without DMRS coordination, and adding such requirements increases complexity considerably.